Those who are easily offended by broad satire aimed at easy targets would be wise to avoid Tropic Thunder. When early trailers of the film showed Robert Downey, Jr., in blackface, that seemed a likely target for uproar and indignation. Instead, it is a scene in the movie in which the R word is used liberally that has sparked protest and a boycott on behalf of the mentally disabled.

The joke is, of course, that writer/director Ben Stiller isn't actually making fun of the cognitively challenged in that scene, but rather actors and assorted Hollywood types who are cognitively, emotionally, psychologically, ethically and physically stunted human beings who will do anything to make a buck or win a coveted award. And so there's plenty of offense to go around in Tropic Thunder: African Americans, Jews, Asians, Method actors, movie executives, drug addicts, the overweight, the hirsute and the recently deceased can all get in line - but nothing in this movie is meant to be taken seriously, and the glancing blows delivered have all the force of powderpuffs.

The contentious scene features Downey's Kirk Lazarus, a white Australian Method actor so committed to his craft that he has a skin-darkening procedure to play the role of a black soldier. Lazarus explains to Stiller's Tugg Speedman - an action star who made a few too many sequels and lost his audience - why his performance as a buck-toothed, mentally retarded man in a movie called Simple Jack was not Oscar-worthy. The underlying joke is that Speedman isn't very smart. This joke is delivered by a very serious man in blackface, which gives the whole scene an extra absurdist zing. Lazarus explains why Forrest Gump is Oscar bait and I Am Sam isn't - and methinks he's actually onto something there.

The scene is one of the less extreme in a movie of extreme comedy; and like most of Stiller's comedy, it is meant to cause discomfort. Stiller's comedy is mostly about humiliation -usually the humiliation of Stiller's own character (and that happens here) - but that humiliation is supposed to make the audience cringe a little bit, too. There's plenty to cringe about in Tropic Thunder, and most of that cringing is accompanied by laughter.

Lazarus and Speedman have their informative talk while trekking through the jungles of Viet Nam. Their director (Steve Coogan) has put them there to shoot his war movie - also called Tropic Thunder - guerrilla-style in order to motivate and/or punish his preening actors. They're accompanied by Jeff Portnoy (Jack Black), a comedian who specializes in fart jokes; a hip-hop entrepreneur named Alpa Chino (Brandon T. Jackson), who shills for an energy drink called Booty Sweat; and minor supporting actor Kevin Sandusky (Jay Baruchel), whose major contribution to the movie is a scene in which he is disemboweled.

The disemboweling of Sandusky is kid stuff in a movie in which hands are blown off and shredded, heads roll and are stuck on rifle barrels, and then the dangly bits are...oh, never mind. The point is, if Stiller were a murderer instead of a movie director, he would shoot a guy, stab him, cut off his limbs, decapitate him, kick the head around and then poke him with a big stick to make sure that he's really, really dead. Since he's a movie director instead, he kinda does the same thing to a joke: He makes sure that it is really and truly dead before he's done with it - which is funny right up to the point where it isn't anymore, although exactly where that boundary lies may be subjective. Or maybe it isn't. I bet George Carlin knew; but I'm not convinced that Stiller does. Still, Tropic Thunder is funny more often than it isn't.

Downey effectively stomps Method acting into the ground in a role that is much funnier than it really ought to be, all things considered. The presence of Alpa Chino helps: His primary mission is to mock Lazarus, his soul brother pretensions and his faux pigmentation. Despite all that, Downey's Lazarus is quite possibly the most authentic character in the bunch, and pulling off an authentic blackface character is something - although it's not that he comes off as authentically black (whatever that means), but as authentically human. Maybe Downey found the real soul in his fake soul man, but it comes as a surprise that his character manages to be both the most pretentious and the least ridiculous: an actual character among caricatures.

Speaking of caricatures, there must, I suppose, be mention of Tom Cruise's turn as Les Grossman, a grotesque, foul-mouthed, murderous movie executive in a hairy fat suit. It is a funny performance only if hard-bodied Tom Cruise dancing lewdly in a hairy fat suit is inherently funny - which it isn't. So why is Robert Downey in blackface funny when Tom Cruise in a fat suit isn't? Again, George Carlin would probably know the answer.

Tropic Thunder is that kind of movie: randomly, inexplicably and unexpectedly funny, and then, just as randomly, not so funny. It's funny when it's smart, funny when it's smart about actors who are not smart, funny when it's about how dumb movies are and - here's where you're supposed to squirm - funny when it's smart about why audiences fall for dumb movies. The movie kicks off with some very funny fake trailers for movies that are just bad enough to be real, and coming soon to a theatre near you. If there's one thing Hollywood loves more than a movie about the mentally disabled, it's a movie about movies. I smell an Oscar.